Polyurethane spray foams and their methods of manufacture are well known. Briefly, the polyurethane polymer is formed by an exothermic chemical reaction between a polyisocyanate and a polyol. This polymerization reaction is typically catalyzed by tertiary amine catalysts and organometallic catalysts. The resulting polymer becomes foam if it is formulated with a blowing agent, in which case the formation of gas bubbles occurs at the same time as urethane polymerization. If water is used as a chemical blowing agent, gaseous carbon dioxide will be produced by the chemical reaction between isocyanates and water. Alternatively, low boiling point liquids can be added as physical blowing agents that are chemically unreactive, but are vaporized by the heat generated by the polymerization reaction. Furthermore, a properly selected liquefied gas can also be used as a physical blowing agent and it will act as a propellant if it has an adequate vapor pressure at the desired temperature range. Surfactants in the foam forming composition stabilize the growing bubbles (cells) and regulate their size. Gas bubbles in the polymer expand upon reduction of pressure in the system, and remain trapped within the cells of the foam. The initial liquid foam cures to a cellular material ranging from a flexible to a rigid foam.
Typically, low pressure two component polyurethane spray foams are formed from two-component systems, commonly referred to as an “A” side and a “B” side, that react when they are mixed. Component “A” contains a diisocyanate or a polyisocyanate with or without a blowing agent or further additives, and component “B” generally contains gaseous blowing agent/propellant and a polyol pre-mix. The polyol pre-mix contains a polyol having two or more hydroxyl groups, tertiary amine catalysts, organometallic catalysts, liquid blowing agent, and water. The “A”-side and “B” side components may include surfactants and other additives. The two components are packaged and stored in separate containers (pressure rated, such as a cylinder or aerosol can) or stored in separate compartments within the same container. Typically, the components of the “A” side and the components of the “B” side are delivered though separate lines into a dispensing unit, such as an impingement mixing or static mixing type spray gun, at a ratio of around 1:1 by weight. In a system with two separate containers, the two components are kept separate throughout this entire system until they come together in the mixing section of the dispensing unit, such as a mixing nozzle or mixing chamber. When dispensed, the liquid contents come out as frothed foam which reacts and cures to form the cellular polyurethane polymer. The spray foam industry in the United States traditionally regards the isocyanate component as “A” side and the component containing polyol as “B” side. The “A” and “B” designations may be reversed in other areas such as Europe.
The gaseous blowing agents or propellants, hydrofluorocarbons (HFCs), currently used in typical low pressure two component polyurethane spray foams are “third generation” blowing agents. The shelf life of current conventional low pressure polyurethane spray foam compositions containing HFCs is at least 6 months, typically 12 months or longer.
Hydrohaloolefins (HHOs) such as hydrofluoroolefins (HFOs) are being developed as “fourth generation” blowing agents because they have been shown to have less global-warming potential than HFCs. Mandates are proposed or now exist in the United States, Canada and Europe to ban the use of HFCs in spray foam compositions. However, there are challenges in formulating a low pressure two-component foam forming composition containing an HHO that is storage stable (i.e., the composition has desired shelf-life stability). Storage stable systems are those having desired chemical reactivity (e.g., having desired gel time and/or tack free time), and being able to produce foams with quality and performance as designed throughout the expected storage life.